1. Field of the Invention
The present invention generally relates to a liquid-crystal display device, and more particularly, to a liquid-crystal display device displaying an image on a liquid-crystal panel.
2. Description of the Related Art
Currently, an active matrix liquid-crystal display device represented by a TFT (thin film transistor) liquid-crystal panel is expected to spread as a display device for general household TVs and OA apparatuses. This is because such an active matrix liquid-crystal display device can be easily made thin and light compared to a CRT, and at the same time, can display no poorer image than a CRT.
In light of the advantage of being thin and light, the active matrix liquid-crystal display device is expected to be applied, not only to a portable information device such as a note PC, but to various multimedia information devices. Besides, a polysilicon LCD realizing a narrow frame is expected to become prevalent with stronger measures being implemented against an EMI (electromagnetic interference).
A description will be given hereinbelow of a structure of such an active matrix liquid-crystal display device displaying one of the highest-quality images among flat panel displays. FIG. 1 is an illustration showing a structure of the conventional liquid-crystal display device. As shown in FIG. 1, a conventional liquid-crystal display device 11 comprises a TFT substrate 1, TFTs (thin-film transistors) 2, signal lines 3, scanning lines 4, a common electrode substrate 5, a common electrode 6, a liquid-crystal layer 7, an electrode lead-out line 8, a signal-line drive circuit 9, a scanning-line drive circuit 10, and pixel electrodes 22. The TFTs 2, the signal lines 3, the scanning lines 4, the common electrode 6, the pixel electrodes 22, the liquid-crystal layer 7 provided between the pixel electrodes 22 and the common electrode 6, compose a liquid-crystal panel.
FIG. 2 is a plan view showing a panel structure in the liquid-crystal display device 11 shown in FIG. 1. As shown in FIG. 2, switching elements each consisting of the TFT 2, the signal lines 3, the scanning lines 4, and the pixel electrodes 22 each connected to the TFT 2, are formed on the TFT substrate 1. Further, as shown in FIG. 1, the signal-line drive circuit 9 driving the signal lines 3, the scanning-line drive circuit 10 driving the scanning lines 4, and the electrode lead-out line 8 are formed on peripheral parts of the TFT substrate 1. In addition, the common electrode 6 made of an ITO (a transparent electrode) or a color filter is formed on a glass substrate on the common electrode substrate 5.
Also as shown in FIG. 2, the TFTs 2 and the pixel electrodes 22 are formed in the form of a matrix on the TFT substrate 1. The signal line 3 supplies an image signal to the pixel electrode 22 via the TFT2. The scanning line 4 transmits a control signal to a gate of the TFT 2, the control signal turning on/off the TFT 2 which is connected to the pixel electrode 22 so as to regulate writing of data to each pixel. It is noted that a unit like the liquid-crystal display device 11 that drives the signal lines 3 and the scanning lines 4 so as to display an image via the pixel electrodes 22 formed in the form of a matrix is referred to as an “active matrix liquid-crystal display device”.
FIG. 3 shows a cross-sectional structure of the liquid-crystal display device 11 shown in FIG. 1. As shown in FIG. 3, the signal-line drive circuit 9 and other elements are formed on the TFT substrate 1. The liquid-crystal layer 7 is provided between the common electrode 6 formed on the common electrode substrate 5 and the pixel electrodes 22 formed on the TFT substrate 1. The TFT substrate 1 is electrically connected to the common electrode substrate 5 by a transfer 14a. In addition, as shown in FIG. 3, a sealing portion 13 is provided between the TFT substrate 1 and the common electrode substrate 5 outside the transfer 14a. The electrode lead-out line 8 and a protective film 12 are formed on a part of the TFT substrate 1 not covered by the common electrode substrate 5. Signals are transmitted from the electrode lead-out line 8 to external devices by using a cable such as a flexible flat cable.
In the liquid-crystal display device 11 having the above-described structure, the TFTs 2 in a selected row are turned on so that an image-signal voltage applied to the signal line 3 is written to each of the pixel electrodes 22, and the information is retained therein by keeping the electric charge until the next time the row is selected. In this course, the inclination of liquid-crystal molecules is so determined in accordance with the retained information as to regulate the amount of light transmission, enabling a gradation display, etc. Further, for a color display, an RGB color filter is used to mix lights.
The backside of such a liquid-crystal panel as above is provided with a surface light source called backlight. Recently, however, a reflective liquid-crystal panel not requiring this backlight attracts attention in the portable information device technology. The reflective liquid-crystal panel is provided with a layer referred to as a reflective electrode, and displays an image by reflecting externally supplied lights and transmits the lights through a liquid-crystal layer.
The above-described conventional liquid-crystal display device 11 has a problem that, since only an ITO (a transparent electrode) or an insulating layer is provided above the signal-line drive circuit 9 and the scanning-line drive circuit 10 formed on peripheral parts of the TFT substrate 1, noises generated from these drive circuits cannot be reduced, which leads to an EMI (electromagnetic interference) especially in a high-frequency operation.
The same problem occurs with respect to the electrode lead-out line 8 shown in FIG. 1 and FIG. 3.